The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Aircraft turbojet engines generate a significant noise pollution. There is strong demand for reducing this pollution, especially as turbojet engines in use are becoming increasingly powerful. The design of the nacelle surrounding a turbojet engine contributes considerably to noise pollution reduction.
In order to further improve the acoustic performances of aircrafts, the nacelles are equipped with acoustic panels intended to attenuate noises generated by the turbojet engine as well as vibrations of structures.
As is known, an acoustic panel comprises an acoustic resonator, forming a layer called intermediate layer, said acoustic resonator being enveloped by a so-called “inner” skin and a so-called “outer” skin to form said acoustic panel.
An acoustic resonator generally comprises one or more hollow core structures (commonly called “honeycomb” structures) separated by a multi-perforated porous skin called septum.
Such a septum can be formed of a plurality of rectangular septum plates, the edges of which are brought into contact at a junction edge, said septum plates being generally obtained by cutting strips, for example made of aluminum. The septum may also be constituted of a plurality of glass plates. The septum makes it possible to obtain a better sound absorption.
Single acoustic resonators, composed of a single honeycomb core structure whereon a septum can be fixed, are well known.
Double acoustic resonators, composed of at least two honeycomb core structures enveloping a septum (which may for example be glued to one of the two structures or on both structures), are also well known.
An acoustic panel comprises in particular a single or double acoustic resonator as defined above and forming an intermediate layer of said panel.
An acoustic panel may be single and comprise a single acoustic resonator as defined above and coated, on the one hand, on the outer side thereof, with an outer skin fixed to the nacelle, forming an outer layer of the acoustic panel and, on the other hand, on the inner side thereof, with an inner skin fixed on said inner side, forming an inner layer of the acoustic panel.
A double acoustic panel comprises a double acoustic resonator as defined above, on the inner side of which an inner skin is fixed, forming an inner layer of the acoustic panel, and on the outer side of which an outer skin fixed to the nacelle forms an outer layer of said acoustic panel.
The outer skin of the acoustic panel, called “acoustic” skin, is perforated and permeable to air; it is intended to be exposed to the air flow passing through the inner space of the nacelle and generating the noise to be attenuated.
The inner skin of the acoustic panel, called “solid” skin, is not perforated and it is impermeable to air; it is not in contact with the air flow passing through the inner space of the nacelle.
The term “outer” is used when a member is exposed to the air flow to be acoustically attenuated. The term “inner” refers to a member which is not in contact with said air flow passing through the nacelle.
Such panels make it possible to “trap” the noise and therefore attenuate noise emissions towards the outer space of the nacelle.
In a known manner, a honeycomb core structure can be carried out by assembling a plurality of honeycomb units being in the form of rectangular plates joined together by nodal joints. Each honeycomb unit has a plurality of honeycomb cells.
The junction surface constituted by the surface of contact between the nodal joints of a first honeycomb unit and the nodal joints of a second honeycomb unit is called the junction edge of honeycomb units.
The acoustic properties of the acoustic panel, that is to say its rate of absorption of the noise depending on the frequency and sound level of the noise, depend in part on the junction with the septum of the honeycomb units, which form the honeycomb core structure. The junction at the junction edge of honeycomb units is commonly made using a foaming glue, such as FM 410® glue.
During assembling and firing of an acoustic panel, the septum undergoes thermal expansion with a magnitude different from that of other surrounding materials, due to use of different materials. This induces offsets and disbonds between the junction edges of the rectangular septum plates and the junction edges of the honeycomb units.
After firing, it is therefore necessary to make up for these disbonds, which impacts the acoustic performance of the panel, and delays the process of manufacturing such a panel.
A known solution is to prevent matching a junction edge of a septum plate with a junction edge of honeycomb units during the assembly.
However, this solution only allows for minimizing the disbond effect, without preventing it completely, so that repairs remain indispensable after the firing step.